Can''t open invitation or rdp file.

it sounds like you're trying to open a file `*rdp` and it comes from a client that is made by microsoft. You will need the same kind of client but made for a mac. There is one on the app store by microsoft so you can I guess pair things. So this isn't really an issue with Mac's RDP.

Like MrHoffman said you could try CoRD. Also try using microsofts client since it is after all the program you're supposed to be using to open that rdp file. Google has a lot of articles in regards to this but be warned. This rabbit hole may go pretty far with not much reward.

This is the simplest solution but requires slackchat https://screenhero.com/ you can take over someone else mouse. So basically you need a client that allows Webcam\Screen Casting which is platform agnostic and it can use VNC which both platforms have. WIN!!

http://netstorage.ringcentral.com/guides/meetings_quickstart_guide.pdf Comes close but your mom would have to host it and surrender control. not easy.

There are other options too like https://www.teamviewer.com/en/help/38-Which-operating-systems-are-supported

you have a lot of options just use google to research.

On the other hand a mac mini is pretty affordable, maybe it's time for an upgrade?

So you're trying to connect to and screen share with a Microsoft Windows system? Try using CoRD, and see if you get any further.

With Mac, it's common to use Messages and Screen Sharing, but the Remote Desktop stuff usually means a VPN into the remote system and some other details. There are some previous similar discussions here and here — which will give you some background and ideas on the general issues with remote access, dynamic DNS and networking, though you'll have to deal with the Windows sharing separately.

As an alternative approach, a migration to an iPad can sometimes help folks that are having trouble with the complexity of a Mac OS X or Microsoft Windows system, too. That won't eliminate the problems and it'll be a different UI to learn, but it can be simpler.

Apple Remote Desktop is a product sold by Apple, and I'd guess that's probably not what you're using here. If you're having issues with Microsoft Remote Desktop package, please check with the Microsoft folks for assistance with that — I'm certainly not going to be any good at providing help with Windows itself, either.

If the target user is a Mac and the user is logged into iCloud, then you can use that to request screen sharing with an OS X system. That's not the case here, though.

Alternatively, there are various web sites which download clients which allow this — TeamViewer and WebEx are examples, though there are various other sites offering similar services.

Microsoft does have an RDP client for OS X, but I've not particularly looked into it or its capabilities.

For local use, CoRD works well enough.

But if you want help with remote access into Microsoft Windows systems, I'd suggest checking with the folks that Microsoft and — beyond that RDP client — seeing what they have available, or maybe a more general forum here.

If you can't get the folks over to iOS, then the other option is to use a VM on an OS X system for what Microsoft Windows software is required, and run OS X on the system.